1. Field of the Invention
The present invention relates to a traffic class of an IEEE 802.16/WiBro system, and more particularly, to an unsolicited grant service (UGS) class.
2. Description of the Related Art
An IEEE 802.16 protocol has a physical layer and a medium access control (MAC) layer for a new wireless broadband network.
An IEEE 802.16/WiBro network includes a base station (BS), a subscriber station (SS), an access router (AR), and the like. The IEEE 802.16 protocol is applied between the BS and SS.
FIG. 1 illustrates a structure of a conventional IEEE 802.16/WiBro network. Referring to FIG. 1, the IEEE 802.16/WiBro network comprises an SS 101 that is a user terminal supporting an IEEE 802.16 protocol, a BS 102 that controls and manages a connection with the SS 101, and an AR 103 that transmits traffic received through the BS 102 to an Internet backbone network.
The IEEE 802.16/WiBro network basically provides a variety of quality of services (QoS). A variety of QoS parameters, such as bandwidth, change according to services provided by an application layer of the SS 101. Unlike in a wired network, in the IEEE 802.16/WiBro network, the characteristics of a physical medium, such as a data transmission rate, may rapidly change according to the characteristics and environment of a wireless medium.
In the IEEE 802.16/WiBro network, the SS 101 defines a service class in order to guarantee the QoS. In more detail, the SS 101 defines the service class, such as an unsolicited grant service (UGS) class, an enhanced-real-time polling service (ertPS) class, a real-time polling service (rtPS) class, a non-real-time polling service (nrtPS) class, and a best effort service (BE) class, and defines scheduling according to the service class.
The UGS class provides a real-time data transmission service having a fixed size and a periodic interval. The UGS class supports a real-time uplink service that periodically transmits data having a fixed size, such as T1/E1, and voice over Internet protocol (VoIP) traffic having no silence suppression.
The rtPS class provides real-time bandwidth request and polling, and variable data scheduling and includes video calls, video games, video on demand (VOD), etc. The rtPS class supports a real-time uplink service that periodically transmits data having a variable size, such as MPEG video traffic. RtPS scheduling must satisfy the characteristics of real-time traffic, and support a method of notifying the BS 102 of a bandwidth required by the SS 101. In order to satisfy the above requirements, the BS 102 performs periodic polling with regard to a specific SS. The specific SS on which the polling is performed requests a bandwidth according to an amount required by the specific SS. Hence, although data transmission efficiency between the SS 101 and the BS 102 is optimized, since the bandwidth is expressly requested, the rtPS class has an overhead compared to the UGS class.
The ertPS class provides a real-time data transmission service having a fixed size and a periodic interval, such as a VoIP service. But, there is a silence period of this class application. Thus, SS 101 requires the change in the bandwidth and accordingly manages a bandwidth QoS to BS 102.
The nrtPS class provides a service sensitive to minimum data processing rate compensation and packet loss and includes large volume FTP, multimedia emails, etc. The nrtPS class supports a data stream having a variable size that is insensitive to latency, such as FTP. A scheduling mechanism of the nrtPS class supports bandwidth allocation by polling and by competition.
The BE class provides a fair scheduling and efficient data retransmission service and includes a web browsing email service, a short message transmission service, low speed file transmission, etc. The BE class supports a method of piggybacking a bandwidth request for periodic polling and data transmission, and a method of requesting bandwidth by competition.
The present invention relates to the UGS class. The 802.16 system initializes the UGS service by communicating a message such as DSA-REQ/DSA-RSP between the SS 101 and the BS 102. In particular, the present invention concerns UGS parameters applied by the BS 102 to the SS 101. The UGS parameters include a grant interval, a guaranteed size, etc. The present invention mainly concerns the grant interval.
FIG. 2 is a diagram for explaining an overload exceeding service capacity of the BS 102.
Referring to FIG. 2, the BS 102 provides a service to each of frames 1 through 10. Each frame has a period d and a capacity R. Each of flows 0 through 5 is a unit of a data transmission service such as the UTS class, rtPS class, etc. required by the SS 101. The flows 0, 2, 3, and 4 are UGS flows. The flows 1 and 5 are rtPS flows.
The BS 102 faces an overload where the capacity R is exceeded in the frames 6 and 10. The reason for this will now be described.
The BS 102 first admits a flow according to whether the flow provides a service within the capacity range of the BS 102 at a corresponding point of time.
However, a problem occurs when grant intervals of the UGS flows 0, 2, 3, and 4 meet one another. In detail, an overload may occur where the capacity range required to serve the UGS flows by the BS 102 is exceeded in a frame at a least common multiple (LCM) of grant intervals.
For example, the flow 2 has a grant interval of 4 and thus the BS 102 provides the service every 4 interval frames. The flow 2 receives the service in the frames 2, 6, and 10. The flow 3 has a grant interval of 2 and thus receives the service in the frames 2, 4, 6, 8, and 10. Since the flows 2 and 3 have the grant interval of 2 and 4, respectively, the LCM is 4 so that the overload occurs in the frame 6 and 10.
The overload is subject to the number of flows belonging to the UGS class, making it impossible to provide the QoS in the BS 102.